Absorber tube

ABSTRACT

An absorber tube is provided that has a metal tube and a sleeve tube, which is made of glass and encloses the metal tube such that an annular space is formed between the metal tube and the sleeve tube. The annular space is evacuated and has at least one container filled with protective gas, where the container is a solder-free pressure container.

The invention relates to an absorber tube according to the preamble of claim 1.

Solar collectors may be equipped with a parabolic mirror, for example, which is also referred to as a collector mirror, and used in so-called parabolic trough power plants. In known parabolic trough power plants, a thermal oil is used as heat-transfer medium, which, by means of the solar rays that are reflected by the parabolic mirrors and focused on the absorber tube, can be heated to approximately 400° C. The heated heat-transfer medium is passed through the metal tube and supplied to a vaporization process, by means of which the thermal energy is converted into electrical energy.

The absorber tube is generally composed of a metal tube, which has a radiation-absorbing layer, and a sleeve tube, which encloses the metal tube. The sleeve tube is made of a material that is transparent in the spectral region of solar radiation, preferably being made of glass. The annular space formed between the metal tube and the sleeve tube is generally evacuated and serves to minimize the heat losses at the outer surface of the metal tube and thus to increase the energy input.

Such absorber tubes are known from DE 102 31 467 B4, for example.

As it increasingly ages, the thermal oil used as heat-transfer medium releases free hydrogen, which is dissolved in the thermal oil. The amount of dissolved hydrogen depends, on the one hand, on the thermal oil used and the operating conditions of oil circulation and, on the other hand, also on the amount of water that comes into contact with the thermal oil. Contact with water can occur more frequently particularly due to leakage in heat exchangers. The released hydrogen enters the evacuated annular space as a result of permeation through the metal tube, the permeation rate also increasing with increasing operating temperature of the metal tube. In consequence, the pressure in the annular space increases as well, which results in an increase in thermal conduction through the annular space, which, in turn, leads to heat losses and to a lower efficiency of the absorber tube or of the solar collector.

In order to at least reduce the pressure increase in the annular space and thereby prolong the service life of the absorber tube, the hydrogen entering the annular space can be bound by getter materials. Absorber tubes that are provided with getter materials in the annular space are known from WO 2004/063640 A1, for example. The uptake capacity of the getter materials is limited, however. After the maximum loading capacity has been reached, the pressure in the annular space increases until it is in equilibrium with the partial pressure of the free hydrogen that has entered the annular space from the thermal oil. The hydrogen results in increased thermal conduction in the annular space with the aforementioned detrimental consequences for the efficiency of the solar collector.

Known from DE 10 2005 057 276 B3 is an absorber tube in which inert gas is fed into the annular space when the capacity of the getter material is exhausted.

The inert gas is present in a container that is sealed with solder and is opened from the outside at an appropriate time. As a result, an H₂/inert gas mixture forms in the annular space, the thermal conductivity of which is only slightly greater in comparison to the evacuated state. The accommodation of the container in the vacuum space of the absorber tube necessitates that it be opened in a contact-free manner from the outside. This can be accomplished by melting the solder through heat input. The other possibility consists in opening the container inductively or by heating an intermediate ring in the vicinity of which the container is fixed in place. The drawback of these opening methods is that the heat input cannot be directed specifically onto the solder seal of the container to a sufficient degree, but instead heats all components in the vicinity of the container as well. In particular, when the sleeve tube is made of glass, the juncture of glass and metallic components (glass-metal juncture) is jeopardized.

The position of the container in the sleeve tube has the fundamental drawback that the container is heated by insolation and the solder seal can unintentionally open. Further drawbacks are to be found in the embrittlement of the solder due to hydrogen uptake. Moreover, the complex geometry of the container makes the manufacture of the solder-sealed opening as well as the entire container expensive.

The problem of the invention is to provide an absorber tube having a container that does not have the mentioned drawbacks and can be opened in a simple manner.

This problem is solved by an absorber tube having the features of claim 1.

A pressure container is understood to refer to a closed container that has, in particular, a spherical or cylindrical shape. The pressure container can have a tube. The tube can also have a curved shape depending the available design space.

The pressure container comprises at least an arched bottom. Preferred is a hemispherical bottom. The lid, too, can be arched, in particular also hemispherical in shape.

A solder-free pressure container is understood to be a pressure container that has no solder, no seal with solder, and no sealing material made of solder. A solder-free pressure container is a pressure container having a solder-free seal. Seal parts are components of the pressure container and are likewise solder-free.

The solder is a heat-sensitive material, the melting point of which is far below that of the other material of the pressure container. A solder is understood to refer to a metal alloy that, depending on the specific application, consists of a specific mixture ratio of metals, primarily lead, tin, zinc, silver, and copper. Solder solders together suitable metals and alloys, such as, for example, copper, bronze, brass, tombac, nickel silver, silver, gold, hard lead, zinc, aluminum, as well as iron, by bonding or amalgamating to them on the surface when it is fused and then solidifying upon cooling. This amalgamation of the solder with the metallic workpieces, materials, structural elements, wires, etc. is the prerequisite for a long-lasting, firmly bonded, solid soldered juncture. This solder has the property that its melting point is lower than that of the metallic workpieces to be joined together. Solder-free means that the pressure container has no solder at any place, particularly not at an opening.

The pressure container can have a bottle neck, which has an opening that can be sealed by means of a sealing element in the shape of a plate, for example. The sealing element is preferably fastened to the filled pressure container by means of welding. Preferred welding processes are friction welding, resistance welding, and laser welding.

Another preferred embodiment provides that, at both of its ends, the container has a bottle neck with a sealing element.

The pressure container is completely closed and does not have a prepared opening that is sealed with a heat-sensitive material, such as, for example, is true in the case a solder seal.

The pressure container has at least one opening for filling with protective gas, said opening being sealed with a sealing part. The sealing part may be the lid or the bottom, for example. A preferred sealing part may also be a plate, in particular a round plate.

Rotationally symmetrical structural components have the advantage that they can be fastened to the container opening in a simply sealed manner by means of resistance or friction welding.

The pressure container may be designed in a bottle shape, for example. In this case, the sealing part is fastened to the opening of the bottle neck.

Preferably, the pressure container is made of steel. According to EN 10020, steel is a material whose mass fraction of iron is greater than that of any other element, whose carbon content is generally <2%, and which contains other elements. Steel is resistant to corrosion, impermeable to gas, and mechanically stable, and hence is particularly suitable as a protective gas container.

Preferred types of steel are those that preferably can be deep-drawn, are vacuum-tight, and/or are heat-resistant up to approximately 600° C.

The pressure container can be opened by means of a laser drilling method. Given an appropriate laser power, the pressure container can be opened in a very short time. The method has the advantage that the pressure container can be opened from the outside without heating the other components of the absorber tube and thus damaging them. The laser beam is aimed specifically onto the container, which can be arranged in the annular space below the sleeve tube at any desired spot that can be reached by the laser beam passing through the sleeve tube. Laser drilling is a non-cutting processing method in which sufficient energy is introduced into the workpiece by means of a laser so as to melt and vaporize the material.

The melting point of steel can be adjusted within a wide range up to approximately 1500° C. Therefore, it is possible to adjust the melting point of the container material, including the wall thickness of the pressure container and the laser parameters, to one another, so as to open the container in an optimal manner.

Because of the high melting point of steel, the maximum allowable temperature of the pressure container is higher than that of a container sealed with solder. It is not necessary to protect the pressure container from insolation, for example, which leads to heating of the pressure container.

Preferably, the pressure container is made of stainless steel. Stainless steel refers to alloyed or unalloyed steels having a special degree of purity, such as, for example, steels whose sulfur and phosphorus content does not exceed 0.025% (see EN 10020).

Preferred stainless steels are Material No. 1.4303 (in particular X4CrNi18-12), Material No. 1.4306 (in particular X2CrNi19-11), Material No. 1.4541, Material No. 1.4571.

The pressure container can be arranged at the metal tube or at the sleeve tube by means of a suitable holding device. Preferably, the pressure container is arranged at a structural component joining the metal tube and the sleeve tube. This can be, in particular, an expansion compensating device.

The pressure container can be fastened by welding, for example, preferably by friction welding. Other welding processes, such as, for example, laser welding or resistance welding, can also be employed.

The wall thickness of the pressure container preferably lies at 0.5-1 mm, in particular at 0.6-0.8 mm. The wall thickness can also be less than 0.5 mm, preferably 0.2 to <0.5 mm, in particular 0.45 mm.

The pressure container is filled with a protective gas, such as, for example, an inert gas having a low thermal conductivity. Xenon or krypton is particularly preferred. The pressure in the container at room temperature is preferably 5-10 bars.

The pressure container can be arranged at the metal tube, at the sleeve tube, or at a structural component joining a sleeve tube and a metal tube. When, for example, an expansion compensating device is provided between the sleeve tube and the metal tube, the pressure container is preferably arranged at such an expansion compensating device. This expansion compensating device can have, for example, a bellows and an appropriate connecting element.

The pressure container is fastened to the connecting element, for example, by means of a holder, one or a plurality of holding elements, a holding clip, a holding bracket, or else a mounting plate. Such a mounting plate can also be provided at the metal tube, for example.

Preferably, the holder encloses the pressure container on the side of the pressure container facing the metal tube. The holder preferably has a trough shape.

This embodiment of the holder has the advantage that, to a large extent, the pressure container can be protected from emission of heat from the absorber tube, from defocused impinging solar radiation from the collector mirror and direct insolation. Strong insolation can impair the container material in terms of its strength under certain conditions. Moreover, the gas pressure in the pressure container increases due to an increase in temperature. The two effects can possibly bring about bursting of the pressure container. This problem is reduced by the shielding afforded by the holder.

The material of the pressure container is vaporized or ejected counter to the impinging beam during laser bombardment and is deposited in the annular space of the absorber tube. Once the wall of the pressure container is penetrated, the protective gas can escape. In this process, the material can also deposit on the inner side of the sleeve tube under some circumstances. The still persisting laser bombardment heats the deposit and thus also the sleeve tube. The effect of this heat is to give rise to mechanical strains in the sleeve tube, which can damage the sleeve tube.

Preferably, therefore, an optical element is arranged in the annular space adjacent to the pressure container, this having the advantage that the material of the container that is vaporized or ejected counter to the impinging beam in the direction of the sleeve tube during laser bombardment deposits on this optical element. As a result, this deposit is prevented from forming at the sleeve tube.

The optical element can be arranged at the sleeve tube, at the metal tube, or at the pressure container.

The holders for the optical element can be combined with the holder for the pressure container, for example, or else arranged at a holder for the pressure container.

The optical element is preferably arranged in the region between the pressure container and the sleeve tube. Such an optical element can be a glass plate, in particular a planar glass plate. This glass plate captures the container material and thus protects the sleeve tube.

According to another embodiment, this optical element can also be designed as a lens, in particular as a concave lens, so as to correct the aberrations of the laser beam caused by the sleeve tube.

According to another embodiment, the optical element can be a section of glass tube in which the pressure container is arranged. It is also possible for the glass tube to be processed in one section and provided there, for example, with a planar section or a lens.

Another embodiment provides that the optical element is an aperture. The aperture opening is preferably only slightly greater than the beam diameter of the laser beam. Preferably, the aperture has a circular aperture opening, the diameter of which is preferably 300 μm.

According to another embodiment, the optical element can also be arranged laterally next to the container. In this case, the optical element is preferably a mirror, in particular a deflection mirror. The laser beam is deflected via the deflection mirror onto the container. Because the container material created by laser bombardment disseminates counter to beam direction, it impinges on the mirror and not on the sleeve tube.

Exemplary embodiments will be described in detail on the basis of the drawings.

Shown are:

FIG. 1 a side view of a protective gas container,

FIG. 2 a cross section through an absorber tube according to a first embodiment,

FIGS. 3, 4, and 5 cross sections of absorber tubes according to other embodiments,

FIG. 6 an embodiment of an absorber tube in lengthwise section,

FIGS. 7 to 12 various embodiments with fastening means for the pressure container and the optical element, and

FIG. 13 a trough-shape holder with a pressure container.

Illustrated in FIG. 1 is a pressure container 30 in side view. The pressure container has a bottle-shaped design with a cylindrical jacket 36 and an arched bottom 37. The bottom is illustrated as a hemispherical bottom.

The cylindrical jacket 36 transitions into a bottle neck 38, which has an opening 39. The opening 39 is sealed by means of a sealing element in the form of a round plate 60. The sealing element is fastened to the filled pressure container 30 by means of friction welding, so that a weld seam 62 is created.

Illustrated in FIG. 2 is a cutout of an absorber tube 1. The absorber tube 1 has a metal tube 10, through which heat-exchanger fluid flows and, as described in the introduction, has radiation-absorbing layers.

The metal tube 10 is arranged concentrically in a sleeve tube 20 that is transparent to solar radiation and is made of glass, for example. Formed between the metal tube 10 and the sleeve tube 20 is an annular space 5, which is evacuated. Arranged inside of this annular space is a pressure container 30, which can be fastened to the sleeve tube 20 or to the metal tube 10 by way of a suitable holder (see FIGS. 6-12).

Arranged in the region between the pressure container 30 and the sleeve tube 20 is an optical element in the form of a planar glass plate 40, 42. A laser beam, which impinges on the sleeve tube 20 perpendicularly from above, passes through the sleeve tube 20 and the planar glass plate 42 and then enters the pressure container. During the drilling process, the material of the container is released and deposits on the bottom side of the planar glass plate 42. In this way, container material is prevented from depositing on the sleeve tube 20.

Illustrated in FIG. 3 is another embodiment, in which the optical element 40 is designed as a concave lens 44. The aberrations that arise due to the curvature of the sleeve tube 20, can be compensated for by the lens 44, so that the laser pulse, as provided for, impinges on the container wall.

Provided as an optical element in FIG. 4 is an aperture 46, which has a circular aperture opening 47 that is slightly greater than the diameter of the laser beam 50.

Illustrated in FIG. 5 is another embodiment, in which the optical element is not arranged in the region between the pressure container 30 and the sleeve tube 20, but rather is adjacent, next to the pressure container 30. What is involved here is a mirror 48, which is arranged such that it is employed as a deflection mirror. The laser beam 50, penetrating from the outside, impinges on the mirror 48 and is deflected so that a horizontal beam impinges on the pressure container 30. The ejected material of the container that is created during laser boring of the pressure container 30 deposits on the mirror 48 and thus does not reach the sleeve tube 20.

Because the optical elements are employed only one time when the pressure container is opened, the deposit on the optical elements does not insofar cause any interference. After laser drilling, the protective gas enters the annular space 5 from the container.

Illustrated in FIG. 6 is one end of an absorber tube 1 in sectional view.

Fastened to the free front-side end of the sleeve tube 20 is a transition element 22, which has a collar 23 that is directed radially inward. Arranged in the annular space 5 formed between the sleeve tube 20 and the metal tube 10 is an expansion compensating device 24 in the form of a bellows 25, which is fastened at its outer end 26 to the collar 23 of the transition element 22.

The bellows 25 thus extends below the transition element 22 into the annular space 5 and is fastened at its opposite-lying end to a connecting element 27, which has an annular disc 28 for this purpose. Arranged at this annular disc is the pressure container 30, which is filled with protective gas and is curved correspondingly to the annular disc and extends over a semicircle. Provided between the pressure container 30 and the sleeve tube 20 is an optical element 40 in the form of a glass plate 42. This glass plate can be flat in design or it may also be curved.

Illustrated in FIG. 7 is a perspective drawing of the absorber tube 1 according to FIG. 6, such that the curved design of the pressure container can be seen. Two holding elements 32 are provided at the two ends of the curved pressure container 30, by means of which the container 30 is fastened to the annular disc 28 of the connecting element 27.

Another embodiment is illustrated in FIG. 8. Here, too, the pressure container, which takes the form of a bottle, is fastened to the annular disc 28 by means of a holding clip 33. The pressure container extends parallel to the lengthwise axis of the absorber tube 1.

Another embodiment is illustrated in FIG. 9. The pressure container 30 also extends along the lengthwise axis of the absorber tube 1 and is fastened to the annular disc 28 by a suitable fastening element (not illustrated). Arranged at annular disc 28 is another holder 34, which supports a lens 44 at its end.

Illustrated in FIG. 10 is an embodiment in which the pressure container 30 is situated inside of a glass tube 45. This embodiment has the advantage that the entire pressure container 30 is shielded and the laser beam can be deflected to any point on the pressure container 30.

Illustrated in FIG. 11 is another embodiment in which a mounting plate 35 is fastened to the annular disc 28. The pressure container 30 lies on this mounting plate, which, in addition, supports another holder 49 for the lens 44.

Illustrated in FIG. 12 is an embodiment in which the mounting plate 35 is arranged on the metal tube 10 and accommodates the pressure container 30.

Illustrated in FIG. 13 is a trough-shaped holder 70, which is arranged at the annular disc 28 of the expansion compensating device 24, which is designed as a bellows 25, by means of a fastening element 78. The trough-shaped holder 70 has a bottom wall 72 that faces the metal tube 10. The trough-shaped holder 70 further has side walls 42, which have inwardly curved shielding walls 76 at the upper edge. As a result, the container 30 is nearly completely enclosed, with only a part of the container wall being left free so that the laser beam can be applied there.

The holder 70 serves to accommodate a getter 80.

LIST OF REFERENCE NUMBERS

-   1 absorber tube -   5 annular space -   10 metal tube -   20 sleeve tube -   22 transition element -   23 collar -   24 expansion compensating device -   25 bellows -   26 outer end -   27 connecting element -   28 annular disc -   29 fastening collar -   30 pressure container -   32 holding element -   33 holding clip -   34 holding bracket -   35 mounting plate -   36 cylindrical jacket -   37 arched bottom -   38 bottle neck -   39 opening -   40 optical element -   42 glass plate -   44 lens -   45 glass tube -   46 aperture -   47 aperture opening -   48 mirror -   49 holder -   50 laser beam -   60 sealing element -   62 weld seam -   70 trough-shaped holder -   72 bottom wall -   74 side wall -   76 shielding wall -   78 fastening element -   80 getter 

1-20. (canceled)
 21. An absorber tube comprising: a metal tube; a sleeve tube that encloses the metal tube, the sleeve being made of material transparent to solar radiation; and an annular space formed between the metal tube and the sleeve tube, the annular space being evacuated; at least one container filled with protective gas in the annular space, wherein the at least one container is a solder-free pressure container.
 22. The absorber tube according to claim 21, wherein the solder-free pressure container has an opening that is sealed with a sealing part.
 23. The absorber tube according to claim 22, wherein the sealing part and the solder-free pressure container are composed of a common material.
 24. The absorber tube according to claim 23, wherein the sealing part is welded to the solder-free pressure container.
 25. The absorber tube according to claim 24, wherein the sealing part is friction welded to the solder-free pressure container.
 26. The absorber tube according to claim 24, wherein the sealing part is laser or resistance welded to the solder-free pressure container.
 27. The absorber tube according to claim 21, wherein the solder-free pressure container is steel.
 28. The absorber tube according to claim 21, wherein the solder-free pressure container has an arched bottom.
 29. The absorber tube according to claim 21, wherein the solder-free pressure container has a wall thickness of 0.5 mm to 1 mm.
 30. The absorber tube according to claim 21, wherein the solder-free pressure container has a wall thickness of 0.2 mm to less than 0.5 mm.
 31. The absorber tube according to claim 21, wherein the solder-free pressure container is arranged at the metal tube or at the sleeve tube.
 32. The absorber tube according to claim 21, further comprising a structural component joining the metal tube and the sleeve tube, wherein the solder-free pressure container is arranged at the structural component.
 33. The absorber tube according to claim 21, further comprising at least one optical element arranged in the annular space adjacent to the solder-free pressure container.
 34. The absorber tube according to claim 33, wherein the at least one optical element is arranged at a location selected from the group consisting of at the sleeve tube, at the metal tube, and at the solder-free pressure container.
 35. The absorber tube according to claim 33, wherein the at least one optical element is arranged in a region between the solder-free pressure container and the sleeve tube.
 36. The absorber tube according to claim 33, wherein the at least one optical element is a glass plate.
 37. The absorber tube according to claim 33, wherein the at least one optical element is a glass tube in which the solder-free pressure container is arranged.
 38. The absorber tube according to claim 33, wherein the at least one optical element is an aperture.
 39. The absorber tube according to claim 33, wherein the at least one optical element is arranged laterally next to the solder-free pressure container.
 40. The absorber tube according to claim 39, wherein the at least one optical element is a mirror. 